ABSTRACT The long-term goals of the proposed research are to elucidate the transcriptional mechanisms regulating neuronal morphogenesis and connectivity in the mammalian brain. We recently discovered that the transcriptional regulator SnoN1 plays an essential role in the control of neuronal positioning in the mammalian brain. Knockdown of SnoN1 by RNAi in postnatal rat pups robustly triggers the excessive migration of granule neurons in the cerebellar cortex in vivo. Remarkably, SnoN1 forms a complex with the transcription factor FOXO1 that represses transcription of the X-linked lissencephaly gene doublecortin (DCX) and thereby controls neuronal positioning in the cerebellar cortex. Importantly, FOXO knockdown phenocopies and DCX knockdown suppresses the SnoN1 knockdown-induced excessive migration of granule neurons in vivo. Interestingly, the SnoN1-related alternatively spliced isoform, SnoN2, opposes the function of SnoN1 in the regulation of FOXO-dependent transcription and neuronal positioning in vivo. These findings define the SnoN1-FOXO1 complex as a novel cell-intrinsic mechanism that orchestrates neuronal positioning. Our findings have also raised several fundamental questions on the mechanisms and biological role of the SnoN1- FOXO1 complex in the control of neuronal positioning. To address these questions, we propose to test the hypothesis that proteins that specifically associate with SnoN1 but not SnoN2 regulate the functions of the SnoN1-FOXO1 complex in transcription and neuronal positioning in vivo. We will also identify novel gene targets of the SnoN1-FOXO1 complex, besides DCX, that mediate the ability of the SnoN1-FOXO1 complex to control neuronal positioning. We will also test the hypothesis that SnoN1-regulation of neuronal positioning is coordinated with other key aspects of neuronal development including neuronal branching and dendrite development in vivo. Finally, because DCX controls neuronal migration in the cerebral cortex, we will us in vivo RNAi and a complementary knockout approach to test the hypothesis that components of the SnoN1 pathway regulate neuronal positioning in the cerebral cortex in vivo. The proposed research represents an important set of experiments that will advance our understanding of the mechanisms that control neuronal positioning in the brain. Since disturbances of neuronal positioning play a critical role in the pathogenesis of inherited mental retardation and epilepsy disorders, elucidating the mechanisms that govern neuronal positioning should also lead to a better understanding of these neurodevelopmental disorders of cognition and epilepsy.